Hi all.Can anyone tell me if I use royal purple rear end fluid that says it's good for posi rear ends,do I still need gm posi additive.I have an eaton posi with 3.73 gears.Thanks in advance for your help.

 

I have an Eaton as well . Its been a few years , but I think what I did as add one small bottle of that slip additive and I think royal purple claimed to have that stuff already in it . I broke in the posi with regular fluid then put in royal purple when it was suggested I change it . I like royal purple diff fluid , so much so I wanted some for my trans , but read people say it's no good . oh well

 

Although I don't have a posi, I still think every diff with a clutch or cone posi system should be using a posi additive. It's always nice to upgrade to a 75w90 synthetic oil while you're at it but the synthetic oil by itself normally isn't good enough for a posi.

Royal Purple does claim that the limited slip additive is already added to their Max Gear synthetic oil so according to them, you shouldn't have to add any more. Their regular 80w90 oil doesn't have the additive.

I run all Lucas oils in my race car.

 

Back in 2010 with 58k on the odometer. I put royal purple 75w90 with gm limited slip additive # 1052358. its still OK with 88k and has been daily driven all year round.